Directing an optical instrument such as binoculars toward an intense light source such as the sun or a welding arc can be very hazardous to the eyes. In astronomical instruments, used to view an object field containing the sun, this problem is sometimes solved by placing, in the focal field of the instrument, an opaque disk positioned exactly at the focused image of the sun and thus blocks out the solar rays. In energy detection devices such as the ones described in U.S. Pat. No. 3,020,406 by T. R. Whitney and U.S. Pat. No. 3,714,430 by R. C. Finvold, a photochromic layer is placed at the focal plane of the device in order to protect sensitive photodetectors. The high energy rays causes the photochromic material or, equivalently system with a photochromic function, to increase in opacity at those points where the rays are most intense. Thus high intensity rays are attenuated by the same dark spots they generate, and dim rays are left unaffected. These prior devices make use of photochromic materials such as germanium or gallium arsenide, which are sensitive to infrared radiation. Theses devices are specifically designed to protect inanimate energy detectors and transducers.
Three other patents by G. S. Levy, extend the state of the art to the protection of the human eye. In U.S. Pat. No. 5,351,151, Levy describes the use of microlens array optics to achieve extreme compactness in optical instruments. His approach results in planar optics capable of selectively eliminating bright light sources. In U.S. Pat. Nos. 5,541,762 and 5,708,522, Levy describes the use of conventional optics, and in particular the application of prisms and mirrors in the design of compact antiglare devices. His devices fall into two main categories depending on whether the photochromic system is transmissive or reflective.
As shown in FIG. 1a, when the photochromic system 11 is transmissive, it is simply placed in the focal plane of the antiglare device. This layer darkens only at those spots where intense light is focused 12, and remains clear elsewhere 13. Since bright objects generate bright spots on the real image projected on the focal plane, the layer becomes dark at precisely those locations and therefore, bright rays 14 self attenuate. Dim rays 15 which traverse clear sections of the layer are left unaffected. The result is a clear field of view with only those bright spots attenuated. It is important for the bright objects to be sharply focused on the photochromic layer so that the mask they generate is well delineated. A practical implementation of such a concept is shown in FIG. 1b where the photochromic layer 16 is placed between the prisms 17 which fold the optical path to reduce the size of the device. FIG. 1c provides a three-dimensional view of the prism assembly for the left eye, showing the photochromic layer 18.
When the photochromic system is reflective, it is placed in the focal plane of the device as shown in FIGS. 2A and 2b. In this case, however, a trade-off must be made: either, as shown in FIG. 2a, the axes 21 and 22 of both incoming and outgoing rays are made perpendicular to the photochromic layer but they cannot be coincident and therefore a significant portion of the image is cut off; or, as illustrated in FIG. 2b, the axes 23 and 24 are made to coincide at the photochromic layer but they cannot be perpendicular to it, and therefore it is much more difficult to obtain a proper focusing of the bright light sources on that system. Without proper focusing the opaque mask generated by bright rays does not match up perfectly well with those rays and therefore does not stop them completely.
It is an object of this invention to provide an antiglare device using a photochromic system that operates in reflective mode and in which the optical axis of the device is perpendicular to the photochromic layer, thus achieving proper focusing of the bright images on the photochromic system.
It is another object of this invention to provide an antiglare device that can be used in welding visors or goggles for eye protection against light generated by an electric arc
It is yet another object of this invention to provide an antiglare device that can be used in binoculars, viewing scopes, rifle scopes, periscopes, and rearview mirrors for eye protection against high intensity light such as solar glare.
It is yet another object of this invention to provide an antiglare device that can be used in still as well as video cameras, to protect their photosensitive material from damage due to direct solar exposure, and to provide them with automatic contrast reduction and wider dynamic range.
This invention describes an antiglare device in which a photochromic system is placed at the focal plane of an optical system Bright rays create, at the focal plane, bright spots which force the photochromic systems to become dark at those same spots and therefore to attenuate the bright rays. The result is a clear field of view where only the bright objects ware dimmed. The photochromic system is maintained perpendicular to the optical axis by means of asymmetrical optical elements such as a polarizing beam splitter and half lenses, to allow the sharp focusing of the real image on the photochromic system, and in turn, the sharp delineation of the dark mask generated by the photochromic layer.